1. Field of the Invention
This invention relates generally to safety systems for preventing explosions in internal turret mooring systems where risers which are carrying hydrocarbons from subsea wells are connected to lines leading to process facilities. In particular the invention relates to an atmosphere control system for preventing explosions in such a mooring system.
2. Description of the Prior Art
In the past, ventilation has been the basis for preventing explosion due to leaks between risers and surface equipment of a turret mooring system. Ventilation systems have inherent difficulties in that explosion potential can remain unacceptably high under certain conditions.
Systems and methods based on the principle of filling an enclosure with inert gas are known in the art of safety systems for marine vessel cargo tanks and in land hydrocarbon storage tanks. Inert gas systems used on marine vessel cargo tanks are described in a book, Inert Gas Systems, International Maritime Organization (IMO), 1990. Guidelines are provided which apply to inert gas system on tankers, particularly to cargo tankers for hydrocarbons. The guidelines are based on current general practice used in the design and operation of inert gas systems using flue gas from the uptake from the ship""s main or auxiliary boilers, and installed on crude oil tankers and combination carriers. The guidelines provide a method with an inert gas system where the protection against a tank explosion is achieved by introducing inert gas into the tank to keep the oxygen content low and reduce to safe proportions the hydrocarbon gas concentration of the tank atmosphere. It can be determined from flammability diagrams that as inert gas is added to hydrocarbon/air mixtures, the flammable range progressively decreases until the oxygen content reaches a level generally taken to be about 11% by volume, below which point no mixture can burn. There are three methods of replacement of gas in cargo tanks, namely: inerting, purging, and gas-freeing. The general policy of cargo tank atmosphere control is that tankers fitted with inert gas systems should have their cargo tanks kept in a nonflammable condition at all times. In line with that policy, tanks should be kept in the inert condition whenever they contain cargo residues or ballast. The oxygen contents should be kept at 8% or less by volume with a positive gas pressure in all the cargo tanks. The atmosphere within the tank should make the transition from the inert condition to the gas-free condition without passing through the flammable condition. In practice this means that before any tank is gas-freed, it should be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line. When a ship is in a gas-free condition before arrival at a loading port, tanks should be inerted prior to loading.
A second inerting method and system is described in a publication, NFPA 69: Standard on Explosion Prevention Systems, of the National Fire Protection Association (NFPA), 1997. The standard described in this publication applies to systems and equipment used for the prevention of explosions by the prevention or control of deflagrations (i.e., combustion with velocities less than the speed of sound).
The standard outlines the minimum requirements for installing systems for the prevention of explosions in enclosures that contain flammable concentrations of flammable gases, vapors, mists, dusts, or hybrid mixtures. Recognized techniques are grouped into two classes in the standard: one based on preventing combustion; the other based on preventing or limiting damage after combustion occurs.
One method of the standard for preventing combustion provides for oxidant concentration reduction which is a technique for maintaining the concentration of the oxidant (e.g. oxygen) in a closed space below the concentration required for ignition to occur. The technique for oxidant concentration reduction for deflagration prevention can be considered for application to any system where a mixture of oxidant and flammable material is confined to an enclosure within which the oxidant concentration can be controlled. The system is maintained at an oxidant concentration low enough to prevent a deflagration by using a purge gas (e.g., inert gas such as nitrogen). Flammability diagrams for specific flammable gases or vapors are used as a basis for determining the level of limiting oxidant concentrations (LOC).
U.S. Pat. No. 5,564,957 discloses an arrangement for dynamically positioning a vessel with thrusters and connecting a riser buoy in a lower receiving module at a submerged place at the bottom of the hull of the vessel. The buoy has an outer buoyant portion anchored to the sea bed by anchor legs. The outer portion of the buoy is locked to the vessel. An inner part of the buoy is rotatably mounted centrally of the outer part. A riser runs from the sea bed to the central part of the buoy which can be removably secured to a flow line of the vessel which leads to storage holds. A long vertical shaft runs from the vessel deck to the connection of the riser at the top of the central part of the buoy to the vessel flow line. Inert gas and ventilation are applied to the shaft from the inert gas and ventilation system of the vessel. Further the shaft at its upper end is provided with a shutter for closing the shaft. The shaft and the upper part of the receiving space can thereby be filled with inert gas (after removal of water) as a safety precaution prior to start of transfer of combustible or inflammable fluids.
Ventilation is also employed for atmosphere control in closed chambers for combustible concentration reduction by mixing and diluting combustible gas in air, followed by removal of the chamber atmosphere mixture via exhausting to the natural atmosphere on topsides of the vessel. This presupposes that combustible gas is present, as in the case of an accidental leak (i.e., upon confirmed detection of the presence of the combustible gas). Ventilating, either continuously or on demand (i.e., upon confirmed detection of gas), is intended to reduce the combustible gas concentration low enough (i.e., below the LEL of the gas) to prevent the formation of a flammable atmosphere. A disadvantage of ventilation for atmosphere control is that, unless the ventilation is designed to deliver a very high number of air changes per hour, even a moderate hydrocarbon release rate may be sufficient to overwhelm the ventilation system and result in a combustible gas concentration between the LEL and UEL (i.e., the flammable range), the atmosphere is potentially flammable, thereby increasing the probability of an explosion. Although for very large releases, the combustible gas concentration could pass through the flammable range quicker, thereby reducing the probability of an explosion, the problem of exhausting the gas after a leak has been controlled still presents a hazard, since ventilating with air would require the atmosphere to pass through the flammable range again. It is desirable therefore to ensure that, regardless of the characteristics of a gas leak within the closed chamber, the atmosphere within the closed chamber will not pass through the flammable range either during the leakage or during clearing of the leaked combustible from the closed chamber.
A disadvantage of employing continuous ventilation for atmosphere control within the QCDC room is that moist sea air is introduced into the atmosphere of the room, allowing for accelerated corrosion and subsequent degradation of critical equipment and instrumentation (e.g., ESD valves and actuators). The effects of corrosion and degradation are compounded in terms of increased risk by the increased potential for leaks from degradation over the life of the equipment. The necessity for more frequent maintenance and repair to control corrosion and degradation creates increased exposure of personnel to hazards as work is conducted within the QCDC room. Also, since more frequent maintenance and repair is needed, the potential for human error is increased. Continuous ventilation (while diluting the combustible/air mixture sufficient to maintain a combustible concentration below the LEL) may actually mask a small hydrocarbon leak, and therefore would not allow detection and correction of the leak before the situation worsens. Any appreciable sized hydrocarbon leak would overwhelm the ability of the ventilation system to dilute the combustible/air mixture sufficiently to maintain a combustible concentration below the LEL. Even if the ventilation system is shut down upon confirmed gas detection at 60% LEL, a high pressure gas release could itself present a static electricity hazard and ignite the gas in the presence of oxygen as the gas concentration passes through the flammable range. Additionally, a ventilation system running continuously at very high air interchange rates provides a potential for ignition sources posed by its metal parts. Further, a ventilation system running continuously at high volume consumes a significant amount of energy, thus adding significantly to the operational costs of the turret mooring system.
A primary object of this invention is to provide an improved atmosphere control system using inert gas principles for an internal turret mooring system based on the NFPA 69 standard described above.
Another object of the invention is to provide an atmosphere control system based on inerting principles which significantly reduces the risk of explosion in a mooring turret, because preventing the formation of a flammable mixture eliminates the probability of ignition.
Another object of the invention is to provide an inerting system for a turret moored FPSO, as opposed to a ventilation system, in order to provide lower capital and operating costs in a relatively simple design, the effectiveness of which relies only on the availability of a continued supply of nitrogen and maintaining an enclosure integrity.
It is also an object of the present invention to provide a structure with a substantially closed chamber within which leakage of a flammable medium may occur and to maintain the gaseous atmosphere of the chamber in a nonflammable condition by introduction of an inert gas such as nitrogen, flue gas, carbon dioxide or the like sufficient to render the chamber atmosphere oxidant defficient.
It is another object of the present invention to provide a QCDC chamber within an internal turret mooring system and a chamber inerting system for ensuring the presence within the chamber of a non-flammable atmosphere even under circumstances where flammable hydrocarbons may exist by leakage from production risers and also having the capability of changing the atmosphere of the chamber to provide for the safe presence therein of maintainance workers.
It is also an object of the present invention to provide a QCDC chamber within an internal turret mooring system which is designed to be maintained at a positive environment pressure, above atmospheric pressure to minimize the potential for oxidant intrusion into the chamber.
It is an even further object of the present invention to provide a QCDC chamber within an internal turret mooring system which is designed to sustain a predetermined leakage developed overpressure and to vent excessive pressure in the event a high volume leak should be developed within the chamber.
Oxidant concentration reduction in the QCDC room could be achieved by mixing and diluting the oxidant (oxygen present in air) by introducing an inert gas (e.g., nitrogen), followed by removal of this atmosphere by purging to the natural atmosphere on topsides. This method renders and maintains an atmosphere nonflammable, regardless of the combustible gas concentration, thereby completely eliminating the potential for combustion. Inerting of a substantially closed atmosphere in this manner eliminates the need for continuous ventilation, and thus eliminates potential introduction of moist sea air into the closed QCDC room so that degradation of the equipment and instrumentation therein by corrosion is minimized. In fact, a continuously inerted enclosure would actually inhibit the corrosion normally expected from an air atmosphere within the QCDC room. This would reduce the inspection frequency and reduce maintenance activities (and manning levels), as well as any risk associated with these activities. It is anticipated that the capital and operating costs of an atmosphere control system employing inerting according to the principles of the present invention would be significantly less than a continuous ventilation type atmosphere control system. The oxidant concentration reduction method of the present invention is based on the inherently safe principal of xe2x80x9cattenuationxe2x80x9d, i.e., using materials under less hazardous conditions. In this case, the attenuation strategy is physical (i.e., dilution) rather than chemical. Thus the preventative method of inerting is preferable to the mitigation method of continuous ventilation.
Briefly, the objects identified above and other features and advantages of the present invention are incorporated in a novel arrangement of an enclosure, referred to as a Quick Connect/Disconnect Room (QCDC) defined by a roof and wall section of the turret and by a wall section and floor of the top of a spider buoy. Surface safety valves, piping, and instrumentation that are critical to isolating hydrocarbon inventories between subsea equipment and the turret, which represent potential leak sources, are located in the enclosure or QCDC room and thus are potential sources of leakage of combustibles into the QCDC room. It is of course desirable that combustible leakage does not occur within the QCDC room, but if it does, it is highly desirable that the combustible leakage be prevented from presenting a danger of explosion.
The enclosure and associated vent ducts and ancillary equipment for the enclosure provide several functions. The enclosure ducts and ancillary equipment provide a space and system for filling and maintaining a volume of inert gas to displace oxygen and prevent formation of a flammable atmosphere when production from the subsea wells to the vessel via the turret is on-line. The enclosure and associated equipment serve as a secondary containment facility in case of a gas leak, with the capability to vent hydrocarbon gas to the atmosphere to prevent overpressure of the enclosure. The enclosure and associated equipment further provide a work area for service personnel that can be adequately ventilated to provide a safe working atmosphere when occupied by personnel to perform maintenance after production is shut down.
By controlling the atmosphere differently for various operating modes, the enclosure and associated vents and ancillary equipment provide flexibility in operation, yet allow for the inherently safe feature of maintaining a nonflammable atmosphere at all times, thereby significantly reducing risk of explosion. It should be borne in mind that the term xe2x80x9cinert gasxe2x80x9d from the standpoint of the present invention, shall mean a pure inert gas or a substantially inert gas that can contain small percentages of other gases, including oxygen, but when introduced into an enclosure in the presence of air and a combustible gas, will reduce the oxidant content of the enclosure gas mixture sufficiently that combustion of the mixture will not occur, regardless of the volume of combustible gas within the mixture.